Market demand for low cost, low environmental impact, and safe microwavable containers for use in food and other packaging applications represents an opportunity for polypropylene (“PP”) to replace polystyrene. However, common linear polypropylene grades display low melt strength and lack strain hardening behavior needed for foams. A polypropylene resin (see WO 2014/070386) with broad molecular weight distribution has shown some promise in terms of improved melt strength, but its strain hardening could be improved. To expand the applications of high melt strength polypropylene resins the inventors have worked towards developing a new polypropylene product with high melt strength and strain hardening through a reactive extrusion process that is compatible with common production technologies. Part of such work involves development of the melt extrusion process, especially reactive extrusion of polypropylenes with organic peroxides and ways to improve such processes. Thus, there is a need for an improved melt extrusion process to form high melt strength and strain hardened polypropylene. The inventors have surprisingly found that the higher processing temperatures impart favorable properties to the polypropylene.
Other relevant publications include EP 2 720 954 A1; EP 2 751 194 A1; EP 2 679 630 A1; EP 2 000 504 A1; EP 1 159 309 B1; U.S. Pat. Nos. 5,180,751; 5,883,151; 6,323,289; 6,573,343; 6,875,826; US 2003/0157286; WO 1997/49759; WO 1999/27007; WO 1994/005707; WO 2012/049690; and WO 2014/070386; and                N. Spisakova et al., in 15 J. MACRM. SCI. & APP. CHEM. 37 (2000);        M. H. Wagner et al., “The strain-hardening behaviour of linear and long-chain-branched polyolefin melts in extensional flows,” in 39 RHEOL. ACTA 97-109 (2000);        R. P. Lagendijk et al., in “Peroxydicarbonate modification of polypropylene and extensional flow properties,” in 42 POLYMER 10035-10043 (2001);        M. Ratzsch et al., 27 PROG. POLYM. SCI. 27 1195 (2002);        A. D. Gotsis, “Effect of long branches on the rheology of polypropylene,” in 48(4) J. RHEOLOGY 895-914 (2004);        P. lacobucci, “High melt strength polypropylene through reactive extrusion with Perkadox 24L,” SPE POLYOLEFINS CONFERENCE, Houston, Tex. (February 2004);        P. Spitael et al., in “Strain hardening in polypropylenes and its role in extrusion foaming,” in 44(11) POLY. ENG. & SCI. 2090-2100 (2004);        K. Jayaraman et al., “Entangling additives enhance polypropylene foam quality,” in SPE PLASTICS RESEARCH ONLINE (2011); and        H. Pol et al., “Microstructure and rheology of high-melt-strength poly-(propylene) impact copolymer,” in SPE PLASTICS RESEARCH ONLINE (2014).        